1. Field of the Invention
The present invention relates to a controllable two-phase network with amplitude compensation.
2. Description of the Related Art
One conventional modulation method for information technology is amplitude modulation. Amplitude-modulation signals have two side bands. The two side bands carry the same information, that is to say one side band is redundant. One side band is, therefore, normally removed from the modulated signal. One possible way to remove a side band is to use a filter. However, owing to the stringent damping and gradient requirements for the filter, a second method is often used, the phase method. In the phase method, the signal and local oscillator signal (LO signal) are split into two signal elements which are phase-shifted through 90° and applied to separate modulators and/or demodulators. Their output signals are added or subtracted and thus produce the upper or lower side band, respectively (see, for example, O. Zinke, H. Brunswig, “Hochfrequenztechnik 2” [Radio-frequency technology 2], fifth Edition, pages 550 to 554, Springer-Verlag, Berlin, 1999).
In order to achieve optimum suppression of one side band while the other side band is unchanged, it is necessary inter alia, for the two LO signals to be phase-shifted through exactly 90°, and to have exactly the same amplitudes. Controllable two-phase networks are used to carry out the method, and these use an LO signal as the input signal to produce two output signals which are of the same amplitude but have a phase difference of 90°. These are used for so-called 0°/90° LO signal processing by IQ modulators and demodulators.
All-pass networks are preferably used for shifting the phase of the LO signal, since they have a frequency-independent amplitude response. Networks which contain few active elements, or even no active elements at all (for example operational amplifiers), are used for radio-frequency signals. Active elements are frequency-limited and adversely affect the all-pass character of a network at high frequencies. For purely passive all-pass networks, only networks which are independent of the characteristic impedance are of interest for this application. These can be set to an exact phase shift of 90° by variation of one parameter, for example the value of a trimming capacitance C or of a trimming resistance R. However, networks such as these require abnormal impedances for the source (ZQ=0) and for the load (ZL→∞).
Passive all-pass networks which are independent of the characteristic impedance are known and are used for side band suppression. As a result of the restrictive conditions relating to the source impedance and load impedance, attempts have been made until now to approximate the abnormal impedances for the source (ZQ=0) and for the load (ZL→∞) as well as possible. As the frequencies rise, the source impedance and load impedance abnormalities become even more difficult to approximate. The phase difference of 90° can admittedly be appropriately readjusted, but there always remains an amplitude error between the two output signals, owing to the finite source and load impedances, and this error detracts from the side band suppression. Variable non-reactive resistances or capacitances are used to control the variable phase difference. A PIN diode is in this case normally used to achieve a variable resistance, and a capacitance diode is normally used to achieve a variable capacitance.